In many applications of laser light, it is essential that the light intensity delivered by the laser be as constant as possible. A certain fluctuation of the intensity around its mean value is, however, unavoidable. One reason is that the energy supply rate from the pump source is never completely steady. There is a noise contribution which the laser not only transfers to its output intensity, but rather tends to exaggerate, especially if it is operated not too far above its threshold. On the other hand, there are sources for fluctuations of the output intensity that are located inside the laser itself. The most common of these internal noise sources is a small vibration of the optical components (e.g. mirrors), such as are caused by acoustic waves in the environment. Some types of lasers have additional sources for fluctuations; dye jet lasers, for example, are easily perturbed by small fluctuations in the flow of the jet.
All these noise contributions can be kept within limits by a proper construction. It is common practice to use regulated power supplies and set up lasers on vibration-damping tables. Nevertheless, a typical laser still has a fluctuation of its output intensity of the order of a few percent. For many applications, this is far too much, and in these cases active stabilization is required.
Active stabilization makes use of a measurement of the output intensity by steering a small portion of the output beam to a detector. The AC component of the detector signal represents the fluctuation that has to be removed and can thus serve as the error signal. All known schemes for active stabilization proceed according to one of the two following schemes: (1) the error signal is fed back to the energy source of the laser, which may be an electric power supply unit, and feedback is arranged in such a way that the error signal is reduced, that is, so the output intensity is stabilized; (2) the error signal controls a variable light attenuator, which may be an electro-optic modulator placed between the laser output and the place where the output intensity sample is taken, and feedback is arranged so as to give the desired effect of reducing the fluctuation.
Both prior art schemes, however, have severe disadvantages. The first scheme may not be applicable at all if there is no way of controlling the power supply. Even if the supply can be reasonably controlled, very often this control can act only very slowly so that only a small bandwidth of the noise is affected. The second scheme avoids these difficulties, but trades them in for others. In order to be effective, the variable attenuator typically attenuates the transmitted beam by 30 to 50 percent. This means that a substantial amount of the total power has to be sacrificed. It is often difficult to produce just enough laser power for a given purpose, and such a sacrifice is then unacceptable.